1. Field of the Invention
The present invention relates to antenna systems in general, and to splashplate antennae in particular.
2. Brief Description of Related Developments
Splashplate antenna systems, which can operate as both receiving and transmitting antennae, normally contain a parabolic-shaped main reflector and a feed assembly. The feed assembly for a xe2x80x9csplashplatexe2x80x9d type antenna at least includes a sub-reflector sometimes referred to as a xe2x80x9csplash platexe2x80x9d or xe2x80x9csplashplate,xe2x80x9d located at one end of a circular waveguide (xe2x80x9cwaveguidexe2x80x9d) that is open in front of the splashplate, and a waveguide xe2x80x9cTxe2x80x9d or an orthomode transducer attached to the other end of the waveguide for processing primary antenna beams. Received signals are reflected from the main reflector to the sub-reflector (splashplate) and then to the waveguide. Transmitted signals generated by the transmission circuitry attached thereto are converted to circularly polarized waves, carried to the splashplate by the circular waveguide and reflected from the sub-reflector and the main reflector to form a secondary antenna beam.
While sub-reflector feed antennae types such as Cassegrain and Gregorian are superior to splashplate feed antennae in electrical performance, the splashplate types are preferable in many systems because of their light weight (facilitating rapid scan operations), relative low construction cost, and low blockage of primary antenna aperture because the sub-reflectors can be relatively small. It is therefore desirable to solve some of their problems. For example, many splashplate feed antennae produce poor illumination of both the sub-reflector and the parabolic reflector that results in a high spillover of antenna beams and lower efficiency. Additionally, such antennae may also generate sidelobes with undesirable energy content, as well as unacceptably high levels of cross-polarization of the electrical and magnetic fields of the transmitted beams.
The reader is referred to FIGS. 1 and 2 which in general show splashplate antenna geometries and which specifically illustrate the geometry and feed configuration of an antenna based on the present invention. In a typical splashplate antenna feed the waveguide steps and the splashplate would be smaller than shown in FIG. 2 and the fields radiating from the waveguide would exhibit a power pattern such as shown in FIG. 3 that is based on the TE11 mode circular field pattern shown in FIG. 4. It will be apparent to those skilled in the art that the wide beamwidth and the curved field patterns shown in FIGS. 3 and 4 produce both significant spillover of the beam at the splashplate and cross-polarization in the radiated beam.
The larger feed and splash plate shown in FIG. 2 solve these problems by supporting both the TE11 and TM11 waveguide modes.
Given the above, it is desirable to provide a splashplate antenna system which embodies the aforementioned advantages sans the aforementioned disadvantages.
In view of the above-identified problems and limitations of the prior art, the present invention provides, in a feeder antenna system, a method of generating secondary antenna beams. The method at least includes the steps of providing a waveguide with multiple cross-sections of different diameters along a propagation axis, and providing a splashplate placed near the waveguide having multiple cross-sections of different diameters normal to the direction of the axis of propagation through the waveguide. The method further at least includes the steps of converting input waveguide signals into multiple waveguide modes, to create Gaussian-like field intensities, and radiating waves from the waveguide against the splashplate, wherein the waves reflected by the splashplate to a main reflector retain their polarization characteristics.
The present invention also provides a splashplate antenna at least including a main reflector, a feed waveguide coupled at a first end to the main reflector and at a second end to the feed horn and splashplate. The waveguide at least includes internally, a series of different dimensioned cross-sections along the axis of propagation causing input signals to be converted into multiple waveguide modes, to create Gaussian-like field patterns. The splashplate at least includes a series of normal faces with different dimensioned cross-sections causing waves received from the waveguide to retain their polarization pattern and to optimally illuminate the main reflector.